This invention relates to a magnetic device which requires the supply of a high flux density used for a magnetic roller, particularly for a plain paper copier.
In conventional magnetic rollers, a magnetic roller is well known wherein a sintered ferrite magnet having a rectangular cross-sectional area is adhered on a shaft by an aligned distribution with special form. However, such an arrangement significantly increases the manufacturing cost to form a shaft of special shape. In addition, the sintered ferrite magnet used is brittle and difficult to assemble. Further, defects can be caused by mechanical impact or vibration occurring after assembly. Furthermore, it is difficult to form a magnet having a profiled cross section because of the molding characteristics of sintered ferrite magnet material and the freedom in design of magnetic poles is very low. A magnet having a construction in the form of a plastic magnet in a bar shape, aligning it so that the direction of the magnetic poles thereof may be in the radial direction with respect to the central axis of the roller, and assembling it, is well known. However, in that case, it is very difficult to obtain a desired and sufficient magnetic flux density and the adjustment of the flux density is also different.
Furthermore, though a method of after-work is also provided in order to increase the magnetic flux density, it is not easy to accomplish.
Even in the case wherein a sintered ferrite magnet is employed, efforts to obtain a higher flux density are performed. In spite of such efforts, since a profiled magnet cannot be obtained when sintered ferrite is used, a method of assistant magnets is adopted in addition to the main magnets (magnets corresponding to or giving the necessary magnetic poles). However, this method is far more difficult in assembly and results in a considerably higher final cost.